Cells for the electrowinning of aluminium having dimensionally stable metal-based anodes

ABSTRACT

A cell for the electrowinning of aluminium comprising one or more anodes ( 10 ), each having a metal-based anode substrate, for instance comprising a metal core ( 11 ) covered with an metal layer  12,  an oxygen barrier layer ( 13 ), one or more intermediate layers ( 14; 14 A,  14 B) and an iron layer ( 15 ). The anode substrate is covered with an electrochemically active transition metal oxide layer, in particular an iron oxide-based outside layer ( 16 ) such as a hematite-based layer, which remains dimensionally stable during operation in a cell by maintaining in the electrolyte a sufficient concentration of iron species and dissolved alumina. The cell operating temperature is sufficiently low so that the required concentration of iron species in the electrolyte ( 5 ) is limited by the reduced solubility of iron species in the electrolyte at the operating temperature, which consequently limits the contamination of the product aluminium by iron to an acceptable level. The iron oxide-based layer ( 16 ) is usually an applied coating or an oxidised surface of a substrate ( 11, 12, 13, 14, 15 ), the surface ( 15 ) of which contains iron.

FIELD OF THE INVENTION

[0001] This invention relates to cells for the electrowinning ofaluminium by the electrolysis of alumina dissolved in a moltenfluoride-containing electrolyte provided with dimensionally stableoxygen-evolving anodes, and to methods for the fabrication andreconditioning of such anodes, as well as to the operation of such cellsto maintain the anodes dimensionally stable.

BACKGROUND ART

[0002] The technology for the production of aluminium by theelectrolysis of alumina, dissolved in molten cryolite, at temperaturesaround 950° C. is more than one hundred years old.

[0003] This process, conceived almost simultaneously by Hall andHéroult, has not evolved as many other electrochemical processes.

[0004] The anodes are still made of carbonaceous material and must bereplaced every few weeks. During electrolysis the oxygen which shouldevolve on the anode surface combines with the carbon to form pollutingCO₂ and small amounts of CO and fluorine-containing dangerous gases. Theactual consumption of the anode is as much as 450 Kg/Ton of aluminiumproduced which is more than ⅓ higher than the theoretical amount of 333Kg/Ton.

[0005] Metal or metal-based anodes are highly desirable in aluminiumelectrowinning cells instead of carbon-based anodes. As mentionedhereabove, many attempts were made to use metallic anodes for aluminiumproduction, however they were never adopted by the aluminium industry.

[0006] EP Patent application 0 306 100 (Nyguen/Lazouni/Doan) describesanodes composed of a chromium, nickel, cobalt and/or iron basedsubstrate covered with an oxygen barrier layer and a ceramic coating ofnickel, copper and/or manganese oxide which may be further covered withan in-situ formed protective cerium oxyfluoride layer.

[0007] Likewise, U.S. Pat. Nos. 5,069,771, 4,960,494 and 4,956,068 (allNyguen/Lazouni/Doan) disclose aluminium production anodes with anoxidised copper-nickel surface on an alloy substrate with a protectivebarrier layer. However, full protection of the alloy substrate wasdifficult to achieve.

[0008] U.S. Pat. No. 4,999,097 (Sadoway) describes anodes forconventional aluminium electrowinning cells provided with an oxidecoating containing at least one oxide of zirconium, hafnium, thorium anduranium. To prevent consumption of the anode, the bath is saturated withthe materials that form the coating. However, these coatings are poorlyconductive and have not found commercial acceptance.

[0009] U.S. Pat. No. 4,504,369 (Keller) discloses a method for producingaluminium in a conventional cell using anodes whose dissolution into theelectrolytic bath is reduced by adding anode constituent materials intothe electrolyte, allowing slow dissolution of the anode. However, thismethod is impractical because it would lead to a contamination of theproduct aluminium by the anode constituent materials which isconsiderably above the acceptable level in industrial production. Tolimit contamination of the product aluminium, it was suggested to reducethe reduction rate of the dissolved constituent materials at thecathode, by limiting the cathode surface area or by reducing masstransfer rates by other means. However, the feasibility of theseproposals has never been demonstrated, nor was it contemplated that theamount of the anode constituent materials dissolved in the electrolyteshould be reduced.

[0010] U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian) describesmetal anodes for aluminium electrowinning coated with a protectivecoating of cerium oxyfluoride, formed in-situ in the cell orpre-applied, this coating being maintained by the addition of smallamounts of cerium to the molten cryolite electrolyte so as to protectthe surface of the anode from the electrolyte attack. All other attemptsto reduce the anode wear by slowing dissolution of the anode with anadequate concentration of its constituents in the molten electrolyte,for example as described in U.S. Pat. No. 4,999,097 (Sadoway) and U.S.Pat. No. 4,504,369 (Keller), have failed.

[0011] In known processes, even the least soluble anode materialreleases excessive amounts constituents into the bath, which leads to anexcessive contamination of the product aluminium. For example, theconcentration of nickel (a frequent component of stable anodes) found inaluminium produced in laboratory tests at conventional cell operatingtemperatures is typically comprised between 800 and 2000 ppm, i.e. 4 to10 times the acceptable level which is 200 ppm.

[0012] The extensive research which was carried out to develop suitablemetal anodes having limited dissolution did not find any commercialacceptance because of the excessive contamination of the productaluminium by the anode materials.

OBJECTS OF THE INVENTION

[0013] A major object of the invention is to provide an anode foraluminium electrowinning which has no carbon so as to eliminatecarbon-generated pollution and increase the anode life.

[0014] A further object of the invention is to provide an aluminiumelectrowinning anode material with a surface having a highelectrochemical activity for the oxidation of oxygen ions for theformation of bimolecular gaseous oxygen and a low solubility in theelectrolyte.

[0015] An important object of the invention is to reduce the solubilityof the surface layer of an aluminium electrowinning anode, therebymaintaining the anode dimensionally stable without excessivelycontaminating the product aluminium.

[0016] Another object of the invention is to provide operatingconditions for an aluminium electrowinning cell under which conditionsthe contamination of the product aluminium is limited.

[0017] A subsidiary object of the invention is to provide a cell for theelectrowinning of aluminium whose side walls are resistant toelectrolyte, thereby allowing operation of the cell without formation ofa frozen electrolyte layer on the side walls and with reduced thermalloss.

SUMMARY OF THE INVENTION

[0018] The invention relates to dimensional stabilisation ofoxygen-evolving anodes of cells for the electrowinning of aluminium bythe electrolysis of alumina dissolved in a molten fluoride-containingelectrolyte. It has been found that dissolution of anodes comprising atransition metal-based oxide surface, in particular an electrochemicallyactive outside layer of iron oxide, cobalt oxide, nickel oxide orcombination thereof, can be kept dimensionally stable duringelectrolysis by maintaining in the electrolyte a sufficientconcentration of dissolved alumina and transition metal species whichare present as one or more corresponding transition metal oxides in theanode surface, and operating the cell at a sufficiently low temperatureso that the required concentration of the transition metal species inthe electrolyte is limited by the reduced solubility thereof in theelectrolyte at the operating temperature, which consequently limits toan acceptable level the contamination of the product aluminium by thetransition metals which are present as one or more correspondingtransition metal oxides in said outside layer.

[0019] The invention is particularly but not exclusively concerned withiron oxide-containing electrochemically active anode surfaces and willbe further described and illustrated with particular reference thereto.

[0020] It has been observed that iron oxides and in particular hematite(Fe₂O₃) have a higher solubility than nickel in molten electrolyte.However, in industrial production the contamination tolerance of theproduct aluminium by iron oxides is also much higher (up to 2000 ppm)than for other metal impurities.

[0021] Solubility is an intrinsic property of anode materials and cannotbe changed otherwise than by modifying the electrolyte compositionand/or the operating temperature of a cell.

[0022] Laboratory scale cell tests utilising a NiFe₂O₄/Cu cermet anodeand operating under steady conditions were carried out to establish theconcentration of iron in molten electrolyte and in the product aluminiumunder different operating conditions.

[0023] In the case of iron oxide, it has been found that lowering thetemperature of the electrolyte decreases considerably the solubility ofiron species. This effect can surprisingly be exploited to produce amajor impact on cell operation by limiting the contamination of theproduct aluminium by iron.

[0024] The solubility of iron species in the electrolyte can even befurther reduced by keeping therein a sufficient concentration ofdissolved alumina, i.e. by maintaining the electrolyte as close aspossible to saturation with alumina. Maintaining a high concentration ofdissolved alumina in the molten electrolyte decreases the solubilitylimit of iron species and consequently the contamination of the productaluminium by cathodically reduced iron.

[0025] Thus, it has been found that when the operating temperature ofaluminium electrowinning cells is reduced below the temperature ofconventional cells an anode coated with an outer layer of iron oxide canbe made dimensionally stable by maintaining a concentration of ironspecies and alumina, in the molten electrolyte sufficient to suppressthe dissolution of the anode coating but low enough not to exceed thecommercially acceptable level of iron in the product aluminium.

CELLS AND OPERATION

[0026] The invention provides a cell for the electrowinning of aluminiumby the electrolysis of alumina dissolved in a molten fluoride-containingelectrolyte. The cell comprises one or more anodes, each having ametal-based substrate and an electrochemically-active iron oxide-basedoutside layer, in particular a hematite-based layer, which remainsdimensionally stable by maintaining in the electrolyte a sufficientconcentration of iron species and alumina. The cell operatingtemperature is sufficiently low so that the required concentration ofiron species in the electrolyte is limited by the reduced solubility ofiron species in the electrolyte at the operating temperature, whichconsequently limits the contamination of the product aluminium by ironto an acceptable level.

[0027] In the context of this invention:

[0028] a metal-based anode means that the anode contains at least onemetal in the anode substrate as such or as an alloy, intermetallicand/or cermet.

[0029] an iron oxide-based layer means that the layer containspredominately iron oxide, as a simple oxide such as hematite, or as partof an electrically conductive and electrochemically active double ormultiple oxide, such as a ferrite, in particular cobalt, manganese,nickel, magnesium or zinc ferrite.

[0030] More generally, the iron-oxide may be present in theelectrochemically active layer as such, in a multi-compound mixed oxide,in mixed crystals and/or in a solid solution of oxides, in the form of astoichiometric or non-stoichiometric oxide.

[0031] The solubility of iron species in the electrolyte may beinfluenced by the presence in the electrolyte of species other thaniron, such as aluminium, calcium, lithium, magnesium, nickel, sodium,potassium and/or barium species.

[0032] Usually, the iron oxide-based outside layer of the anode iseither an applied layer or obtainable by oxidising the surface of theanode substrate which contains iron as further described below.

[0033] The cell is usually operated with an operating temperature of theelectrolyte below 910° C. The operating temperature of the electrolyteis usually above 700° C., and preferably between 820° C. and 870° C.

[0034] The electrolyte may contain NaF and AlF₃ in a weight ratioNaF/AlF₃ from about 0.74 to 0.82. The concentration of alumina dissolvedin the electrolyte is usually below 8 weight %, usually between 2 weight% and 6 weight %.

[0035] In order for the produced aluminium to be commerciallyacceptable, the amount of dissolved iron in the electrolyte whichprevents dissolution of the iron oxide-based anode layer is such thatthe product aluminium is contaminated by no more than 2000 ppm iron,preferably by no more than 1000 ppm iron, and if required by no morethan 500 ppm iron.

[0036] The cell may comprise means for periodically or intermittentlyfeeding iron species into the electrolyte to maintain the requiredamount of iron species in the electrolyte at the operating temperaturewhich prevents the dissolution of the iron oxide-based anode layer. Themeans for feeding iron species may feed iron metal and/or an ironcompound, such as iron oxide, iron fluoride, iron oxyfluoride and/or aniron-aluminium alloy.

[0037] The means for feeding iron species may periodically feed ironspecies together with alumina into the electrolyte. Alternatively, themeans for feeding iron species may be a sacrificial electrodecontinuously feeding iron species into the electrolyte.

[0038] The dissolution of such a sacrificial electrode may be controlledand/or promoted by applying a voltage thereto which is lower than thevoltage of oxidation of oxygen ions. The voltage applied to thesacrificial electrode may be adjusted so that the resulting currentpassing through the sacrificial electrode corresponds to a currentnecessary for the dissolution of the required amount of iron speciesinto the electrolyte replacing the iron which is cathodically reducedand not otherwise compensated.

[0039] Advantageously, the cell may comprise an aluminium-wettablecathode which can be a drained cathode on which aluminium is producedand from which it continuously drains, as described in U.S. Pat. No.5,651,874 (de Nora/Sekhar) and U.S. Pat. No. 5,683,559 (de Nora).

[0040] Usually, the cell is in a monopolar, multimonopolar or in abipolar configuration. Bipolar cells may comprise the anodes asdescribed above as the anodic side of at least one bipolar electrodeand/or as a terminal anode.

[0041] Preferably, the cell comprises means to improve the circulationof the electrolyte between the anodes and facing cathodes and/or meansto facilitate dissolution of alumina in the electrolyte. Such means canfor instance be provided by the geometry of the cell as described inco-pending application PCT/IB99/00222 (de Nora/Duruz) or by periodicallymoving the anodes as described in co-pending application PCT/IB99/00223(Duruz/Bellò).

[0042] The cell according to the invention may also have side wallsprovided with an iron oxide-based outside layer which is during celloperation in contact only with the electrolyte and which is maintaineddimensionally stable by the amount of iron species and alumina dissolvedin the electrolyte. The iron oxide-based layer on the side walls may bein contact with molten electrolyte. By maintaining the side walls freefrom frozen electrolyte, the cell may be operated with reduced thermalloss.

[0043] The invention relates also to a method of producing aluminium ina cell as described hereabove. The method comprises keeping the anodedimensionally stable during electrolysis by maintaining a sufficientconcentration of iron species and alumina in the electrolyte, andoperating the cell at a sufficiently low temperature so that therequired concentration of iron species in the electrolyte is limited bythe reduced solubility of iron species in the electrolyte at theoperating temperature, which consequently limits the contamination ofthe product aluminium by iron to an acceptable level.

CELL COMPONENTS AND METHODS OF FABRICATION

[0044] Another aspect of the invention is an anode which can bemaintained dimensionally stable in a cell as described above. The anodehas a metal-based substrate comprising at least one metal, an alloy, anintermetallic compound or a cermet. The substrate is covered with aniron oxide-based outside layer, in particular a hematite-based layer,which is electrochemically active for the oxidation of oxygen ions intomolecular oxygen.

[0045] As already stated above, the iron oxide-based outside layer ofthe anode is usually either an applied layer or obtainable by oxidisingthe surface of the anode substrate which contains iron.

[0046] The iron oxide-based layer may be formed chemically orelectrochemically and optionally in-situ on the anode substrate.

[0047] Alternatively, the iron oxide-based layer may be applied as acolloidal and/or polymeric slurry, and dried and/or heat treated. Thecolloidal and/or polymeric slurry may comprise at least one of alumina,ceria, lithia, magnesia, silica, thoria, yttria, zirconia, tin oxide andzinc oxide.

[0048] The iron oxide-based layer may also be formed by arc or plasmaspraying iron oxide or iron onto the anode substrate followed by anoxidation treatment.

[0049] The iron oxide-based layer may be formed, or consolidated, byheat treating an anode substrate, the surface of which contains ironand/or iron oxide, in an oxidising gas at a temperature above theoperating temperature of a cell in which the anode is to be inserted.

[0050] Usually, the anode substrate is heat treated in air or in oxygenat a temperature of 950° C. to 1250° C. for a period of time dependingon the temperature.

[0051] The iron oxide-based layer can comprise a dense iron oxide outerportion, a microporous intermediate iron oxide portion and an innerportion containing iron oxide and a metal from the surface of the anodesubstrate.

[0052] The anode substrate may comprise a plurality of layers carryingthe iron oxide-based layer. For instance, the anode substrate may bemade by forming on a core layer an oxygen barrier layer which is coatedwith at least one intermediate layer carrying the iron oxide-basedlayer, the oxygen barrier layer being formed before or after applicationof the intermediate layer(s).

[0053] The oxygen barrier layer may be formed by applying a coating ontothe core layer before application of the intermediate layer(s) or bysurface oxidation of the core layer before or after application of theintermediate layer(s).

[0054] The oxygen barrier layer and/or the intermediate layer may beformed by slurry application of a precursor. Alternatively, the oxygenbarrier layer and/or the intermediate layer may be formed by arc orplasma spraying oxides thereof, or by arc or plasma spraying metals andforming the oxides by heat treatment.

[0055] Usually, the oxygen barrier layer contains at least one oxideselected from chromium, niobium and nickel oxide, and is covered with anintermediate layer containing copper, or copper and nickel, and/or theiroxides.

[0056] A preferred embodiment of the anode is a composite,high-temperature resistant, non-carbon, metal-based anode having ametal-based core structure of low electrical resistance for connectingthe anode to a positive current supply and coated with a series ofsuperimposed, adherent, electrically conductive layers consisting of:

[0057] a) at least one layer on the metal-based core structure forming abarrier substantially impervious to molecular oxygen and also tomonoatomic oxygen;

[0058] b) one or more intermediate layers on the outermost oxygenbarrier layer to protect the oxygen barrier and which remain inactive inthe reactions for the evolution of oxygen gas and inhibit thedissolution of the oxygen barrier; and

[0059] c) an electrochemically-active iron oxide-based outside layer, inparticular a hematite-based layer, on the outermost intermediate layer,for the oxidation reaction of oxygen ions present at theanode/electrolyte interface into monoatomic oxygen, as well as forsubsequent reaction for the formation of biatomic molecular oxygenevolving as gas.

[0060] In some embodiments, the iron oxide layer is coated onto apassivatable and inert anode substrate.

[0061] Different types of anode substrate may be used to carry anapplied iron oxide-based layer. Usually, the anode substrate comprisesat least one metal, an alloy, an intermetallic compound or a cermet.

[0062] The anode substrate may for instance comprise at least one ofnickel, copper, cobalt, chromium, molybdenum, tantalum, iron, and theiralloys or intermetallic compounds, and combinations thereof. Forinstance, the anode substrate may comprise an alloy consisting of 10 to30 weight % of chromium, 55 to 90% of at least one of nickel, cobalt oriron, and 0 to 15% of aluminium, titanium, zirconium, yttrium, hafniumor niobium.

[0063] Alternatively, some iron-containing anode substrates are suitablefor carrying an iron oxide-based layer which is either applied onto thesurface of the anode substrate or obtained by oxidation of the surfaceof the substrate. The anode substrate may for instance contain an alloyof iron and at least one alloying metal selected from nickel, cobalt,molybdenum, tantalum, niobium, titanium, zirconium, manganese andcopper, in particular between 50 and 80 weight % iron and between 20 and50 weight % nickel, preferably between 60 and 70 weight % iron andbetween 30 and 40 weight % nickel.

[0064] Another aspect of the invention is a bipolar electrode whichcomprises on its anodic side an anode as described above and which canbe maintained dimensionally stable during operation in a bipolar cell.

[0065] These anode materials may also be used to manufacture cellsidewalls which can be maintained dimensionally -stable during operationof the cell as described above.

[0066] A further aspect of the invention is a cell component which canbe maintained dimensionally stable in a cell as described above, havingan iron oxide-based outside layer, in particular a hematite-based layer,which is electrochemically active for the oxidation of oxygen ions intomolecular oxygen. The hematite-based layer may cover a metal-based anodesubstrate comprising at least one metal, an alloy, an intermetalliccompound or a cermet.

[0067] Yet another aspect of the invention is a method of manufacturingan anode of a cell as described above. The method comprises forming aniron oxide-based outside layer on a metal-based anode substrate made ofat least one metal, an alloy, an intermetallic compound or a cermeteither by oxidising the surface of the anode substrate which containsiron, or by coating the iron oxide-based layer onto the substrate.

[0068] This method may also be used for reconditioning an anode asdescribed above, whose iron oxide-based layer is damaged. The methodcomprises clearing at least the damaged parts of the iron oxide-basedlayer from the anode substrate and then reconstituting at least the ironoxide-based layer.

VARIATION OF THE INVENTION

[0069] Generally, the teachings and principles disclosed hereaboverelating to anodes, cells and cell operation are also applicable to anyanode whose electrochemically active layer comprises an oxidisedtransition metal, such as an oxidised nickel-cobalt alloy, as describedat the outset of the summary of the invention.

[0070] In particular, nickel-cobalt active anode surfaces may also bekept dimensionally stable by maintaining a sufficient amount ofdissolved alumina and nickel and/or cobalt species in the electrolyte.

[0071] Whereas nickel as well as cobalt on their own are poor candidatesas electrochemically active materials for aluminium electrowinningcells, an alloy of nickel and cobalt shows the following properties.

[0072] A nickel-cobalt alloy forms upon oxidation complex oxides, inparticular (Ni_(x)Co_(1-x)) O, having semi-conducting properties.

[0073] Furthermore, nickel-cobalt oxides provide an advantage overconventional nickel ferrite. Whereas trivalent iron ions of nickelferrite are slowly replaced by trivalent aluminium ions in theoctahedral sites of the spinel lattice, which leads to a loss ofconductivity and of mechanical stability, nickel-cobalt alloys oxidisedin oxygen at 1000° C. lead to a semi-conducting mixed oxide structure ofNiCo₂O₄ and CO₃O₄ spinels which is similar to the NaCl lattice. In thesespinels, a replacement of trivalent cobalt ions by trivalent aluminiumions is unlikely.

[0074] In order to form an electrochemically active layer suitable foraluminium electrowinning anodes, the cobalt nickel atomic ratio ispreferably chosen in the range of 2 to 2.7.

BRIEF DESCRIPTION OF THE DRAWINGS

[0075] The invention will now be described by way of example withreference to the accompanying schematic drawings, in which:

[0076]FIG. 1 is a cross-sectional view through an anode made of an anodesubstrate comprising a plurality of layers and carrying on the outermostlayer the iron oxide-based layer, and

[0077]FIG. 1a is a magnified view of a modification of the appliedlayers of the anode of FIG. 1.

DETAILED DESCRIPTION

[0078]FIG. 1 shows an anode 10 according to the invention which isimmersed in an electrolyte 5. The anode 10 contains a layered substratecomprising a core 11 which may be copper, an intermediate layer 12, suchas electrodeposited nickel, covering the core 11, to provide ananchorage for an oxygen barrier layer 13. The oxygen barrier 13 may beapplied by electrodepositing a metal such as chromium, niobium and/ornickel and heat treating in an oxidising media to form chromium oxide,niobium oxide and/or nickel oxide.

[0079] On the oxygen barrier layer 13 there is a protective intermediatelayer 14 which can be obtained by electrodepositing, arc spraying orplasma spraying and then oxidising either a nickel-copper alloy layer,or a nickel layer and a copper layer and interdiffusing the appliednickel and copper layers before oxidation. The protective intermediatelayer 14 protects the oxygen barrier layer 13 by inhibiting itsdissolution.

[0080] The protective intermediate layer 14 is covered with anelectrodeposited, arc-sprayed or plasma-sprayed iron layer 15 which issurface oxidised to form an electrochemically active hematite-basedsurface layer 16, forming the outer surface of the anode 10 according tothe invention.

[0081] In FIG. 1, the iron layer 15 and the electrochemically activehematite-based surface layer 16 cover the substrate of the anode 10where exposed to the electrolyte 5. However the iron layer 15 and thehematite-based layer 16 may extend far above the surface of theelectrolyte 5, up to the connection with a positive current bus bar.

[0082]FIG. 1a shows a magnified view of a modification of the appliedlayers of the anode 10 of FIG. 1. Instead of a single intermediate layer14 shown in FIG. 1, the anode 10 as shown in FIG. 1a comprises twodistinct intermediate protective layers 14A,14B.

[0083] Similarly to the anode 10 of FIG. 1, the anode 10 of FIG. 1acomprises a core 11 which may be copper covered with a nickel platedlayer 12 forming an anchorage for a chromium oxide oxygen barrier layer13. However, the single oxidised interdiffused or alloyed nickel copperlayer 14 shown in FIG. 1 is modified in FIG. 1a by firstly applying onthe oxygen barrier 13 a nickel layer 14A followed by a copper layer 14B.The nickel and copper layers 14A,14B are oxidised at 1000° C. in airwithout prior interdiffusion by a heat treatment in an inert atmosphere,thereby converting these layers into a nickel oxide rich layer 14A and acopper oxide rich layer 14B. The nickel oxide rich layer 14A and thecopper oxide rich layer 14B may interdiffuse during use in the cell.

[0084] The intermediate layers 14;14A,14B may either be oxidised beforeuse of the anode 10, before or after application of an iron layer 15, orduring normal electrolysis in a cell.

[0085] The intermediate layers 14A,14B of the anode 10 of FIG. 1a arecovered with an electrodeposited, arc-sprayed or plasma-sprayed ironlayer 15 which is surface oxidised to form an electrochemically activehematite-based surface layer 16, forming the outer surface of the anode10 according to the invention.

[0086] The invention will be further described in the followingExamples:

EXAMPLE 1

[0087] Aluminium was produced in a laboratory scale cell comprising ananode according to the invention.

[0088] The anode was made by pre-oxidising in air at about 1100° C. for10 hours a substrate of a nickel-iron alloy consisting of 30 weight %nickel and 70 weight % iron, thereby forming a dense hematite-basedsurface layer on the alloy.

[0089] The anode was then tested at a current density of about 0.8 A/cm²in a fluoride-containing molten electrolyte at 850° C. containing NaFand AlF₃ in a weight ratio NaF/AlF₃ of 0.8 and approximately 4 weight %alumina. Furthermore, the electrolyte contained approximately 180 ppmiron species obtained from the dissolution of iron oxide therebysaturating the electrolyte with iron species and inhibiting dissolutionof the hematite-based anode surface layer.

[0090] To maintain the concentration of dissolved alumina in theelectrolyte, fresh alumina was periodically fed into the cell. Thealumina feed contained sufficient iron oxide so as to replace the ironwhich had deposited into the product aluminium, thereby maintaining theconcentration of iron in the electrolyte at the limit of solubility andpreventing dissolution of the hematite-based anode surface layer.

[0091] The anode was extracted from the electrolyte after 100 hours andshowed no sign of significant internal or external corrosion aftermicroscopic examination of a cross-section of the anode specimen.

[0092] The produced aluminium was also analysed and showed an ironcontamination of about 800 ppm which is below the tolerated ironcontamination in commercial aluminium production.

EXAMPLE 2

[0093] An anode was made by coating by electro-deposition a structure inthe form of a rod having a diameter of 12 mm consisting of 74 weight %nickel, 17 weight % chromium and 9 weight % iron, such as Inconel®,first with a nickel layer about 200 micron thick and then a copper layerabout 100 micron thick.

[0094] The coated structure was heat treated at 1000° C. in argon for 5hours. This heat treatment provides for the interdiffusion of nickel andcopper to form an intermediate layer. The structure was then heattreated for 24 hours at 1000° C. in air to form an oxygen barrier layerof chromium oxide on the core structure and oxidising at least partlythe interdiffused nickel-copper layer thereby forming the intermediatelayer.

[0095] A further layer of a nickel-iron alloy consisting of 30 weight %nickel and 70 weight % having a thickness of about 0.5 mm was thenapplied on the interdiffused nickel copper layer by arc or plasmaspraying.

[0096] The alloy layer was then pre-oxidised at 1100° C. for 6 hours toform a chromium oxide barrier layer on the Inconel® structure and adense hematite-based outer surface layer on the alloy layer.

[0097] The anode was then tested in molten electrolyte containingapproximately 4 weight % alumina at 850° C. at a current density ofabout 0.8 A/cm². The anode was extracted from the cryolite after 100hours and showed no sign of significant internal or external corrosionafter microscopic examination of a cross-section of the anode sample.

[0098] As a variation, the Inconel® core structure can be replaced by anickel-plated copper body which is coated with a chromium layer andoxidised to form a chromium oxide oxygen barrier which can be covered asdescribed above with an interdiffused nickel-copper intermediate layerand the electrochemically active hematite-based outer layer.

1. A cell for the electrowinning of aluminium by the electrolysis ofalumina dissolved in a molten fluoride-containing electrolyte,comprising one or more anodes, each having a metal-based substrate andan electrochemically-active iron oxide-based outside layer, inparticular a hematite-based layer, which remains dimensionally stable bymaintaining in the electrolyte a sufficient concentration of ironspecies and dissolved alumina, the cell operating temperature beingsufficiently low so that the required concentration of iron species inthe electrolyte is limited by the reduced solubility of iron species inthe electrolyte at the operating temperature, which consequently limitsthe contamination of the product aluminium by iron to an acceptablelevel.
 2. The cell of claim 1, wherein the iron oxide-based outsidelayer is either an applied layer or obtainable by oxidising the surfaceof the anode substrate which contains iron.
 3. The cell of claim 2,wherein the anode substrate comprises a plurality of layers carrying theiron oxide-based layer.
 4. The cell of claim 3, wherein the anodesubstrate comprises an electrically conductive core layer covered withan oxygen barrier layer coated with at least one intermediate layercarrying the iron oxide-based layer.
 5. The cell of claim 4, wherein theoxygen barrier layer contains at least one oxide selected from chromium,niobium and nickel oxide.
 6. The cell of claim 4, wherein theintermediate layer contains copper, or copper and nickel, and/or theiroxides.
 7. The cell of claim 1, wherein the anode substrate comprises atleast one metal, an alloy, an intermetallic compound or a cermet.
 8. Thecell of claim 7, wherein the anode substrate comprises at least one ofnickel, copper, cobalt, chromium, molybdenum, tantalum, iron, and theiralloys or intermetallic compounds, and combinations thereof.
 9. The cellof claim 8, wherein the anode substrate comprises an alloy consisting of10 to 30 weight % of chromium, 55 to 90% of at least one of nickel,cobalt or iron, and 0 to 15% of aluminium, titanium, zirconium, yttrium,hafnium or niobium.
 10. The cell of claim 8, wherein the anode substratecontains an alloy of iron and at least one alloying metal selected fromnickel, cobalt, molybdenum, tantalum, niobium, titanium, zirconium,manganese and copper.
 11. The cell of claim 10, wherein the substratealloy comprises 30 to 70 weight % iron and 30 to 70 weight % nickel. 12.The cell of claim 10, wherein the substrate alloy comprises an alloy ofiron and cobalt.
 13. The cell of claim 1, wherein the operatingtemperature of the electrolyte is above 700° C., preferably between 820°C. and 870° C.
 14. The cell of claim 1, wherein the electrolyte containsNaF and AlF₃ in a weight ratio NaF/AlF₃ from 0.7 to 0.85.
 15. The cellof claim 1, wherein the concentration of alumina dissolved in theelectrolyte is below 8 weight %, preferably between 2 weight % and 6weight %.
 16. The cell of claim 1, comprising means for intermittentlyor continuously feeding iron species into the electrolyte to maintain anamount of iron species in the electrolyte preventing the dissolution ofthe iron oxide-based anode layer.
 17. The cell of claim 16, wherein themeans for feeding iron species feeds iron metal and/or an iron compound.18. The cell of claim 17, wherein the means for feeding iron speciesfeeds iron oxide, iron fluoride, iron oxyfluoride and/or aniron-aluminium alloy.
 19. The cell of claim 16, wherein the means forfeeding iron species is arranged to periodically feed the iron speciestogether with alumina into the electrolyte.
 20. The cell of claim 16,wherein the means for feeding iron species is a sacrificial electrodecontinuously feeding the iron species into the electrolyte.
 21. The cellof claim 20, wherein the sacrificial electrode is connected to a currentsupply arranged to apply a voltage which is lower than the voltage ofoxidation of oxygen O— and supply a current controlling and/or promotingthe dissolution of the sacrificial electrode into the electrolyte. 22.The cell of claim 1, comprising an aluminium-wettable cathode.
 23. Thecell of claim 22, comprising a drained cathode.
 24. The cell of claim 1,which is in a bipolar configuration.
 25. The cell of claim 1, comprisingmeans to improve the circulation of the electrolyte between the anodesand facing cathodes and/or means to facilitate dissolution of alumina inthe electrolyte.
 26. An anode which can be maintained dimensionallystable in a cell for the electrowinning of aluminium according to claim1, having a metal-based substrate comprising at least one metal, analloy, an intermetallic compound or a cermet, the substrate beingcovered with an iron oxide-based outside layer, in particular ahematite-based layer, which is electrochemically active for theoxidation of oxygen ions into molecular oxygen.
 27. The anode of claim26, wherein the iron oxide-based outside layer is either an appliedlayer or obtainable by oxidising the surface of the anode substratewhich contains iron.
 28. The anode of claim 27, wherein the ironoxide-based layer comprises a dense iron oxide outer portion, amicroporous intermediate iron oxide portion and an inner portioncontaining iron oxide and a metal from the surface of the anodesubstrate.
 29. The anode of claim 27, wherein the anode substratecomprises a plurality of layers carrying the iron oxide-based layer. 30.The anode of claim 29, wherein the anode substrate comprises anelectrically conductive core layer covered with an oxygen barrier layercoated with at least one intermediate layer carrying the ironoxide-based layer.
 31. The anode of claim 30, wherein the oxygen barrierlayer contains at least one oxide selected from chromium, niobium andnickel oxide.
 32. The anode of claim 30, wherein the intermediate layercontains copper, or copper and nickel, and/or their oxides.
 33. Theanode of claims 26, wherein the anode substrate comprises at least oneof nickel, copper, cobalt, chromium, molybdenum, tantalum, iron, andtheir alloys or intermetallic compounds, and combinations thereof. 34.The anode of claim 33, wherein the anode substrate comprises an alloyconsisting of 10 to 30 weight % of chromium, 55 to 90% of at least oneof nickel, cobalt or iron, and 0 to 15% of aluminium, titanium,zirconium, yttrium, hafnium or niobium.
 35. The anode of claim 33,wherein the anode substrate contains an alloy of iron and at least onealloying metal selected from nickel, copper, cobalt, chromium,molybdenum, tantalum, iron, and their alloys or intermetallic compounds,and combinations thereof.
 36. The anode of claim 33, wherein the alloysubstrate comprises 30 to 70 weight % iron and 30 to 70 weight % nickel.37. The anode of claim 33, wherein the substrate alloy comprises analloy of iron and cobalt.
 38. A bipolar electrode which comprises on itsanodic side an anode according to claim
 26. 39. A method ofmanufacturing an anode according to claim 26, said method comprisingforming an iron oxide-based outside layer, in particular ahematite-based layer, on a metal-based anode substrate made of at leastone metal, an alloy, an intermetallic compound or a cermet either byoxidising the anode surface of the substrate which contains iron, or bycoating the iron oxide-based layer onto the substrate.
 40. The method ofclaim 39, wherein the iron oxide-based layer is formed chemically orelectrochemically on the anode substrate.
 41. The method of claim 39,wherein the iron oxide-based layer is applied as a colloidal and/orpolymeric slurry, and dried and/or heat treated.
 42. The method of claim41, wherein the colloidal and/or polymeric slurry comprises at least oneof alumina, ceria, lithia, magnesia, silica, thoria, yttria, zirconia,tin oxide and zinc oxide.
 43. The method of claim 39, wherein the ironoxide-based layer is formed by arc or plasma spraying iron oxide or irononto the anode substrate followed by an oxidation treatment.
 44. Themethod of claim 39, wherein the iron oxide-based layer is formed, orconsolidated, by heat treating an anode substrate, the surface of whichcontains iron and/or iron oxide, in an oxidising gas at a temperatureabove the operating temperature of the cell in which the anode is to beinserted.
 45. The method of claim 44, wherein the anode substrate isheat treated at a temperature of 950° C. to 1250° C.
 46. The method ofclaim 43, wherein the anode substrate is heat treated in air or inoxygen.
 47. The method of claim 39, wherein the iron oxide-based layeris formed on an anode substrate comprising a plurality of layers. 48.The method of claim 47, wherein the anode substrate is made by formingon a core layer an oxygen barrier layer which is coated with at leastone intermediate layer and the iron oxide-based outside layer, saidoxygen barrier layer being formed before or after application of theintermediate layer(s).
 49. The method of claim 48, wherein the oxygenbarrier layer is formed by applying a coating onto the core layer beforeapplication of the intermediate layer(s) or by surface oxidation of thecore layer before or after application of the intermediate layer(s). 50.The method of claim 49, wherein the oxygen barrier layer and/or theintermediate layer is/are formed by slurry application of a precursor.51. The method of claim 49, wherein the oxygen barrier layer and/or theintermediate layer is/are formed by arc or plasma spraying oxidesthereof, or by arc or plasma spraying metals and forming the oxides byheat treatment.
 52. The method of claim 39, for reconditioning an anodeaccording to claim 26 whose iron oxide-based layer is damaged, themethod comprising clearing at least the damaged parts of the ironoxide-based layer from the anode substrate and then reconstituting atleast the iron oxide-based layer.
 53. A method of producing aluminium ina cell according to claim 1, the cell comprising an anode having ametal-based anode substrate and an iron oxide-based outside layer, inparticular a hematite-based layer, which is electrochemically active forthe oxidation of oxygen ions into molecular oxygen, said methodcomprising keeping the anode dimensionally stable during electrolysis bymaintaining a sufficient concentration of iron species and dissolvedalumina in the electrolyte, and operating the cell at a sufficiently lowtemperature so that the required concentration of iron species in theelectrolyte is limited by the reduced solubility of iron species in theelectrolyte at the operating temperature, which consequently limits thecontamination of the product aluminium by iron to an acceptable level.54. The method of claim 53, wherein the cell is operated at anelectrolyte temperature above 700° C., preferably between 820° C. and870° C.
 55. The method of claim 53, wherein the cell is operated with anelectrolyte containing NaF and AlF₃ in weight ratio NaF/AlF₃ from 0.7 to0.85.
 56. The method of claim 53, wherein the amount of dissolvedalumina contained in the electrolyte is maintained is below 8 weight %,preferably between 2 weight % and 6 weight %.
 57. The method of claim53, wherein the amount of dissolved iron preventing dissolution of theiron oxide-based anode layer is such that the product aluminium iscontaminated by no more than 2000 ppm iron, preferably by no more than1000 ppm iron, and even more preferably by no more than 500 ppm iron.58. The method of claim 53, wherein iron species are intermittently orcontinuously fed into the electrolyte to maintain the amount of ironspecies in the electrolyte which prevents at the operating temperaturethe dissolution of the anode iron oxide-based layer.
 59. The method ofclaim 58, wherein the iron species are fed in the form of iron metaland/or an iron compound.
 60. The method of claim 59, wherein the ironspecies are fed into the electrolyte in the form of iron oxide, ironfluoride, iron oxyfluoride and/or an iron-aluminium alloy.
 61. Themethod of claim 60, wherein the iron species are periodically fed intothe electrolyte together with alumina.
 62. The method of claim 58,wherein a sacrificial electrode continuously feeds the iron species intothe electrolyte.
 63. The method of claim 62, comprising applying avoltage which is lower than the voltage of oxidation of oxygen O— andsupplying an electric current to the sacrificial electrode to controland/or promote the dissolution of the sacrificial electrode into theelectrolyte.
 64. The method of claim 63, comprising adjusting theelectric current supplied to the sacrificial electrode so that itcorresponds to a current necessary for the dissolution of the requiredamount of iron species into the electrolyte replacing the iron which iscathodically reduced and not otherwise compensated.
 65. The method ofclaim 53, for producing aluminium on an aluminium-wettable cathode. 66.The method of claim 65, wherein the produced aluminium continuouslydrains from said aluminium-wettable cathode.
 67. The method of claim 53,for producing aluminium in a bipolar cell according to claim 24,comprising passing an electric current from the surface of the terminalcathode to the surface of the terminal anode as ionic current in theelectrolyte and as electronic current through the bipolar electrodes,thereby electrolysing the alumina dissolved in the electrolyte toproduce aluminium on each cathode surface and oxygen on each anodesurface.
 68. The method of claim 53, comprising circulating theelectrolyte between the anodes and facing cathodes thereby improvingdissolution of alumina into the electrolyte and/or improving the supplyof dissolved alumina under the active surfaces of the anodes.
 69. A cellcomponent which can be maintained dimensionally stable in a cell for theelectrowinning of aluminium according to claim 1, having an ironoxide-based outside layer, in particular a hematite-based layer, whichis electrochemically active for the oxidation of oxygen ions intomolecular oxygen.
 70. The cell component of claim 69, wherein thehematite-based layer covers a metal-based substrate comprising at leastone metal, an alloy, an intermetallic compound or a cermet.
 71. A cellfor the electrowinning of aluminium by the electrolysis of aluminadissolved in a molten fluoride-containing electrolyte, comprising one ormore anodes, each having a metal-based substrate and anelectrochemically-active transition metal oxide-based outside layerwhich remains dimensionally stable by maintaining in the electrolyte asufficient concentration of dissolved alumina and transition metalspecies which are present as one or more corresponding transition metaloxides in the electrochemically-active layer, the cell operatingtemperature being sufficiently low so that the required concentration oftransition metal species in the electrolyte is limited by the reducedsolubility thereof in the electrolyte at the operating temperature,which consequently limits the contamination of the product aluminium toan acceptable level by the transition metal(s) present as one or morecorresponding transition metal oxides in the electrochemically-activelayer.
 72. The cell of claim 71, wherein the electrochemically-activelayer is made of an oxidised nickel-cobalt alloy which remainsdimensionally stable by maintaining in the electrolyte a sufficientconcentration of nickel and/or cobalt species.
 73. The cell of claim 72,wherein the cobalt nickel atomic ratio is in the range of 2 to 2.7. 74.A method of producing aluminium in a cell according to claim 71,comprising an anode having a metal-based anode substrate and anelectrochemically-active transition metal oxide-based outside layer,said method comprising keeping the anode dimensionally stable duringelectrolysis by maintaining a sufficient concentration of dissolvedalumina and transition metal species which are present as one or morecorresponding transition metal oxides in the electrochemically-activelayer, and operating the cell at a sufficiently low temperature so thatthe required concentration of transition metal species in theelectrolyte is limited by the reduced solubility thereof in theelectrolyte at the operating temperature, which consequently limits thecontamination of the product aluminium to an acceptable level by thetransition metal(s) present as one or more corresponding transitionmetal oxides in the electrochemically-active layer.